[X86][SSE] Ensure that vector shift-by-immediate inputs are correctly bitcast to the result type

vXi8/vXi64 vector shifts are often shifted as vYi16/vYi32 types but we weren't always remembering to bitcast the input.

Tested with a new assert as we don't currently manipulate these shifts enough for test cases to catch them.

llvm-svn: 294308